Systems and methods for soft tissue navigation

ABSTRACT

There are provided systems and methods for soft tissue navigation. A sensor unit provides tracking information to determine positional data for a patient&#39;s structural anatomy and gravity direction measurements to determine a direction of gravity relative to the anatomy. A computing unit, during a procedure, computes the direction of gravity and determines spatial information for soft tissues of the patient, responsive to the direction of gravity. Expected soft tissue movement may be provided by a computer model modelling the mechanics of the soft tissues responsive to the direction of gravity, or by other means such as look up table. Surgical navigation data is presented via a display unit. Medical images may be registered and navigation provided relative to the images. Tool positions may be tracked via the sensor unit. Spatial differences in soft tissues between orientations of the patient (relative to gravity) may be determined and presented.

CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No.62/455,851 filed Feb. 7, 2017, which is incorporated herein byreference.

FIELD

The following disclosure relates to computer assisted procedures such ascomputer assisted surgical navigation and localization. Moreparticularly, the disclosure relates to computer systems and methods fornavigation of soft tissues.

BACKGROUND

Surgical navigation has commonly been used to perform accurate,minimally invasive procedures involving soft tissues. Typically, theseare image-guided procedures (i.e. they are with reference to a medicalimage, such as an MRI scan). Example surgeries include taking biopsies,excising tumours, placing implants, etc. A challenge with surgicalnavigation of soft tissues stems from the fact that soft tissues aredeformable, and therefore may move based on any applied forces. One suchforce is gravity.

Additional background for the use of navigation systems in cranialsurgery is discussed in “Evaluation of intraoperative brain shift usingan ultrasound-linked navigation system for brain tumor surgery”, S. Ohueet al, Neurol Med Chir (Tokyo) 50, 291˜300, 2010, the entire content ofwhich is incorporated herein by reference.

SUMMARY

There are provided systems and methods for soft tissue navigation. Asensor unit provides tracking information to determine positional datafor a patient's structural anatomy and gravity direction measurements todetermine a direction of gravity relative to the anatomy. A computingunit, during a procedure, computes the direction of gravity anddetermines spatial information for soft tissues of the patient,responsive to the direction of gravity. Expected soft tissue movementmay be provided by a computer model modelling the mechanics of the softtissues responsive to the direction of gravity, or by other means suchas look up table. Surgical navigation data is presented via a displayunit. Medical images may be registered and navigation provided relativeto the images. Tool positions may be tracked via the sensor unit.Spatial differences in soft tissues between orientations of the patient(relative to gravity) may be determined and presented.

In an example, there is provided a system to perform a surgicalprocedure on soft tissues. The system comprises a sensor unit,comprising an optical sensor to generate optical measurements from atracker and at least one other sensor to provide inclinationmeasurements to determine a direction of gravity, wherein the tracker isconfigured to provide positional information in up to six degrees offreedom to the optical sensor; a reference element to couple with astructural member of a patient's anatomy; and a computing unit. Thecomputing unit is configured to: compute a registration of thestructural member of the patient's anatomy relative to the referenceelement; calculate a direction of gravity relative to the structuralmember of the patient's anatomy based on the inclination measurementsand the registration; determine spatial information of the soft tissuesrelative to the reference element based on the registration, thedirection of gravity, and expected soft tissue movement provided by acomputer model modelling the mechanics of the soft tissues of thepatient's anatomy responsive to the direction of gravity; and providesurgical navigation data for display.

In an example there is provided a system to perform a surgical procedureon soft tissues. The system comprises a sensor unit, comprising anoptical sensor to generate optical measurements from a tracker and atleast one sensor to provide inclination measurements to determine adirection of gravity, wherein the tracker is configured to providepositional information in up to six degrees of freedom to the opticalsensor; a reference element to couple with a structural member of apatient's anatomy; and a computing unit. The computing unit isconfigured to: receive medical image data of the patient's anatomyincluding the soft tissues, the patient's anatomy being in a first knownorientation with respect to gravity during imaging; compute aregistration of the structural member of the patient's anatomy relativeto the reference element; calculate a second orientation with respect togravity of a patient's anatomy based on inclination measurements and theregistration; calculate the orientation difference between the firstknown orientation and the second orientation; and provide theorientation difference for display.

In an example, there is provided a system to perform a surgicalprocedure on soft tissues of a patient's anatomy. The system comprises asensor unit, comprising an optical sensor to generate opticalmeasurements from a tracker and at least one sensor to provideinclination measurements to determine a direction of gravity, whereinthe tracker is configured to provide positional information in up to sixdegrees of freedom to the optical sensor; a reference element to couplewith a structural member of the patient's anatomy; and a computing unit.The computing unit is configured to: compute a registration of thestructural member of the patient's anatomy relative to the referenceelement; calculate a direction of gravity relative to the structuralmember of the patient's anatomy based on inclination measurements andthe registration; provide the direction of gravity to a computer modelmodelling the mechanics of the soft tissues of the anatomy; and receivedata from the computer model regarding a shift of the soft tissues ofthe anatomy. The surgical procedure may be a cranial procedure and theshift may be a shift of a brain.

When a computing unit is described as being “configured to” performcertain operations, it may be configured via instructions stored in astorage device which when executed by one or more processing unitsconfigure the operations of the computing unit or it may be hardwareconfigured such as by an application specific circuit (ASIC) or othercircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of an intra-operative localization system inaccordance with one example configuration where an optical sensor ismounted to a head clamp.

FIG. 2 is a representation of an intra-operative localization system inaccordance with one example configuration where an optical sensor ismounted to an OR cart supporting an intra-operative computing unit.

FIG. 3 is a representation of an intra-operative localization system inaccordance with one example configuration where an optical sensor ishand held. Further a display unit shows a representative graphical userinterface providing a 4-Up view of medical images of a patient's skull.

FIGS. 4A and 4B are representations of a patient skull in twoorientations.

FIG. 5 is a representation of an optical sensor of a localizationsystem, showing selected components internal to the optical sensor, inaccordance with one example configuration.

FIGS. 6 to 8 are flowcharts of respective operations of a computing unitin accordance with examples herein.

DESCRIPTION

Reference in the specification to “one embodiment”, “preferredembodiment”, “an embodiment”, or “embodiments” means that a particularfeature, structure, characteristic, or function described in connectionwith the embodiment is included in at least one embodiment, and may bein more than one embodiment. Also, such phrases in various places in thespecification are not necessarily all referring to the same embodimentor embodiments.

Surgical navigation of soft tissues is provided, including compensationfor the effect of gravity on the position of the soft tissues. Cranialnavigation will be used as the primary exemplary application, althoughit will be appreciated by those skilled in the art that the describedsystems and methods may be applied to other types of surgery involvingsoft tissues.

A cranial navigation system is described in U.S. provisional applicationNo. 62/384,410, filed Sep. 7, 2016 of Hladio et al., and entitled“Systems and Methods for Surgical Navigation, Including Image-GuidedNavigation of a Patient's Head”, formalized as PCT/IB2017/055400 filedSep. 7, 2017, the entire contents of which are incorporated herein byreference. A reference element is attached to the patient's skull (via ahead clamp). The skull provides a structural member of the patient'sanatomy that is rigid. A “structural member” as used herein is a part ofthe patient's anatomy that provides a stable surface (e.g. a bone) thatis not prone to or is less susceptible to deformation due to factorssuch as gravity, pressure, etc. The structural member may also allow thereference element to be attached to it. The brain is a soft tissueorgan; it has a nominal spatial location within the skull, but is freeto move (i.e. shift, deform) to an extent. When attempting to localize asmall region (e.g. a region of interest) in the brain, the movement maybe significant, and should be accounted for. An exemplaryintra-operative localization system 100 is shown in FIG. 1 in accordancewith an example. Additional examples of intra-operative localizationsystems (200 and 300) are shown in FIGS. 2 and 3 respectively.

In FIG. 1, an optical sensor 102 (depicted as a camera) is connected toa workstation 104 (i.e. a computing unit) to provide optical signals forgenerating positional measurements of a tracker 106 visible to thesensor. The tracker is configured to provide positional information inup to six degrees of freedom to the optical sensor 102. The workstation104 executes instructions to provide surgical navigation of a surgicaltool 108 (depicted as a probe coupled to tracker 106) with respect tothe brain of a patient 110. The workstation 104 may comprise or becoupled to a display unit 112 for displaying user interfaces (e.g.screens) which may present measurements, medical images, live videoimages, workflow, etc. with respect to the procedure.

Pre-operative data 114 may be loaded onto the workstation 104.Pre-operative data may include medical images (e.g. brain or otherpatient images) obtained per-operatively from scans of the patient'sanatomy using one or more modalities (e.g. magnetic resonance imaging(MRI)) for display. Pre-operative data may include geometric definitionsof surgical tools or other objects to be tracked. During localization,workstation 104 receives sensor data from sensor 102 and determines apose of the tracker 106. In some examples, a single tracker may beremovable and coupled to different objects having different geometries.In some examples, a tracker is permanently coupled to a particularobject. Workstation 104 may use the geometrical definition of the objectalong with the pose of the tracker to determine positional informationfrom the object such as the pose (location) of a tip of the tool.Pre-operative data 114 may be remotely stored relative to theworkstation 104 such as on a server (represented by cloud 116), on a USBdevice (not shown) or other storage device (not shown). Pre-operativedata 114 may be stored on a hard drive or other storage device (both notshown) of workstation 104. Workstation 104 may have a communicationsystem (not shown) for communicating data wirelessly or in a wiredmanner. The communication system may have one or more interfaces (notshown), for example, to couple the workstation to sensor 102 such as viaa cable 118. Workstation 104 may comprise one or more processing unitsfor performing operations such as may be instructed by software(instructions) stored on a storage device (e.g. memory) coupled to theone or more processing units.

Sensor 102 is coupled to the anatomy of the patient 110 (the patient'sskull) using a head clamp 120 and sensor arm 122 to rigidly couple thesensor to a rigid structure of the patient. A craniotomy 124 receives atip of surgical tool 108. Workstation 104 may track the location of thetip of the tool via tracker 106 even though the tip may not be visiblewhen located within the skull (and brain tissue).

FIG. 2 shows an example of a localization system 200 in which the sensor102 is coupled to an OR cart 202 via a sensor arm 204. In thisconfiguration, the (optical) tracker 106 is coupled to the patient 110via head clamp 120 and a tracker arm 206. The tracker may be removablycoupled to the tracker arm 206 via a kinematic (e.g. magnetic) mount 208to position the tracker in a same position each time it is coupled tothe tracker arm 206. A separate optical tracker (not shown) or opticaltracker 106 may be coupled to a surgical tool (not shown in FIG. 2),which tool may be tracked during a procedure. Provided that the cameraand patient remain stationary following a registration procedure (asdescribed below), tracker 106 may be removed from tracker arm 206 andused on a tool, if desired. FIG. 2 illustrates a head clamp 120 having adifferent configuration than is shown in FIG. 1, which is not materialto the disclosure.

FIG. 3 shows an example of a localization system 300 in which sensor 102may be held in a hand 302 of a user such as a surgeon or other personattending in the OR. The field of view 304 may be directed at thepatient 110 to capture optical measurements of tracker 106. Tracker 106is coupled to the patient's anatomy (e.g. skull) via a tracker mountingstructure 306. A display unit 112 of workstation 104 displays a userinterface (UI) showing medical images (e.g. 308, 310, 312 and 314) in a4-Up view as described further below.

Registering the patient's anatomy to the localization system 100 is astep involving generating a mathematical spatial relationship(registration data) representing the relative position of a referenceelement and the patient's anatomy. The reference element may be a sensor(e.g. the sensor 102 (camera) of FIG. 1), or a tracker 106 (asillustrated in FIGS. 3 and 6). The sensor 102 is used to generateoptical measurements of a tracker attached to the surgical tool 108relative to the reference element that is coupled to a structuralelement of a patient's anatomy. For navigation, one tracker is attachedto a surgical tool 108 (such as the probe of FIG. 1).

The registration step involves receiving inputs (e.g. a user localizinganatomical landmarks or imaging fiducials using a probe for measuring bythe camera) and performing computations based on the inputs to generateregistration data. In some instances, image registration is performed.Image registration entails determining the spatial relationship betweenthe reference element and the image of the patient's anatomy, and isbased on computations correlating inputs received (e.g. user inputs)with image features. Details of such image registration are included inU.S. provisional 62/384,410. Various registration tools (e.g. an axisframe, a probe with a tip, a plane registration device) may be used toperform the registration. Details of such registration tools areprovided in U.S. application Ser. No. 15/425,690, filed Feb. 6, 2017 byFanson et al. titled “Systems, methods and devices for imageregistration and surgical localization”, published May 25, 2017 asUS-2017-0143433-A1 and issued as U.S. Pat. No. 9,713,506 on Jul. 25,2017, which are incorporated herein by reference.

Upon registration and image registration, image guided surgery may beperformed. During such surgery, the position of a surgical tool (e.g.108) relative to a patient's anatomy (e.g. a brain) is depicted relativeto a medical image (e.g. an Mill) of the patient's anatomy. An exemplaryview provided for display to display unit 112, from a computing unit104, is shown in FIG. 3, in which four views of the medical image areprovided. These 4-Up views may include a coronal 308, a transverse 310,a sagittal 312 and an isometric view 314.

In many types of cranial surgery, a head of a patient 110 is immobilizedwithin a head clamp 120, the head being oriented in accordance with asurgical plan, the plan indicating a desired location for accessing thesurgical site (e.g. the location of a craniotomy 124). The orientationof the head (based on the surgical plan) determines the direction ofgravity during the surgical procedure. Gravity will act on the patient'stissues, including the soft tissues of interest (i.e. the brain). Whenoriented for surgery, the head may have an approximately knownorientation with respect to gravity. Furthermore, during the surgicalprocedure, the head's orientation may change intentionally (e.g. due torepositioning), or unintentionally (due to movement of the head clamp120), causing a change in the direction of gravity with respect to thebrain.

In FIGS. 4A and 4B, two respective orientations of a patient's head aredepicted. In each orientation, the same anatomical tissue is depicted: askull 402A and 402B (exemplary of a structural anatomical member), abrain 404A and 404B (exemplary of soft tissues), and a lesion 406A and406B (exemplary of a region of interest in the soft tissues). In eachorientation, the soft tissues (including the region of interest) have adifferent position, based at least in part on the effect of gravity(depicted as force g).

As depicted in FIG. 5, sensor 102 has inclination sensing capabilitiesvia an inclinometer (or accelerometer) integrated into the sensor 102.The sensor 102 comprises an optical sensor (a camera 500 comprising alens 502, an optical window 504, etc.) that is connected by a rigidstructure 506 to an inclinometer 508. The sensor 102 is also connectedvia a connector 510 to cable 118 to couple the sensor 102 to theworkstation 104 (e.g. as in FIGS. 1, 3 and 6). In this case, the sensor102 provides optical and inclination measurements, both related to acommon frame of reference. Further discussion on how to enable this isprovided below.

In order to provide inclination measurements, an inclinometer 508, suchas an accelerometer, may be integrated within the sensor, as shown inFIG. 5. The inclinometer measurements are combined with opticalmeasurements using techniques known in the art of sensor fusion, suchthat measurements are provided with a common frame of reference. A rigidstructure 506 exists between the location of the camera 500 and theinclinometer 508, thus creating a rigid mechanical relationship. Thisrelationship is unique for each physical device and is used by theworkstation 104 when both the inclinometer 508 and camera 500communicate measurements to it by any means, e.g. wired through cable118, wireless, etc. The workstation 104 may use this relationship toalign inclinometer coordinate values to the optical coordinate values,and display further calculations in the same frame of reference.

In addition to or alternative to accelerometers, other sensingcomponents may be integrated to assist in registration and/or poseestimation. Such sensing components include, but are not limited to,gyroscopes, magnetometers, etc. It may be preferable for the sensingcomponents to be in the form of electronic integrated circuits.

The sensor 102 provides measurements of the direction of gravity to theworkstation 104. The workstation 104 is able to calculate the directionof gravity relative to the reference element. In one example (e.g. FIG.1), the sensor 102 is the reference element, and the direction ofgravity is with respect to the sensor 102 by default. In another example(e.g. FIGS. 2 and 3), a tracker 106 is the reference element; in thiscase, the computing unit calculates the direction of gravity relative tothe sensor 102, and calculates the pose of the reference element. Thedirection of gravity relative to the reference element is calculated byexpressing the gravity vector in the frame of reference of the referenceelement (i.e. using spatial mathematics).

The reference element may be coupled to a structural member of thepatient's anatomy, such that any movement of the structural member ofthe patient's anatomy is reflected by substantially the same movement ofthe reference element. The reference element may be rigidly attached tothe structural member of the patient's anatomy; where the structuralmember is bone, the reference element may be attached via one or morebone pins or screws.

The reference element may also be non-invasively attached over apatient's skin, using, for example, adhesives, suction cups, stickers,clamps, elastic bands or other mounting structures. FIG. 3 illustrates anon-invasive reference element (tracker 106) mounted via a trackermounting structure 306, secured to the skull of the patient 110 bymating with their facial features using a glasses-like structure.

Multiple reference elements may be attached to the same structuralelement (not shown). One advantage of using multiple reference elements,is that there is a degree of redundancy, so that if one referenceelement's position with respect to the structural member changes, thiswould be detectable and/or correctable by detection and/or correctionfunctions executing on the computing unit (i.e. where the otherreference element does not move).

Registration of a structural member of the patient's anatomy may beperformed, since it may be difficult and impractical to register thesoft tissues of interest, due to soft tissue movement and difficultyattaching a reference element directly to soft tissues. For example, apatient's skull may be registered, since it is impractical to directlyregister a patient's brain due to the fact that the procedure may beminimally invasive, and only a small part of the brain is exposed. Also,it is not practical to attach a reference element to the brain, whereasattaching a reference element to the skull (via head clamp 120, as shownin FIG. 2 or tracker mounting structure 306 of FIG. 3) is feasible.Furthermore, the brain is expected to move during the procedure.

The soft tissues may have a nominal position relative to the structuralmember of the patient's anatomy. The position of the soft tissues withrespect to the structural member of the patient's anatomy is based onmechanics. Various mechanical factors may influence this relativeposition, including: the direction of gravity; pressure (e.g. if thesoft tissues are within a structural cavity, the pressure within thecavity); blood pressure; tissue material properties, includingproperties of aged or diseased tissues; and, the size and shape oftissues.

A computer model of the mechanics of soft tissues is implemented as asoftware module as described in U.S. publication no. 20050101855 A1published 12 May 2005 of Miga et al., titled “Apparatus And Methods OfBrain Shift Compensation And Applications Of The Same”, the contents ofwhich are incorporated by reference herein. The software module receivesinputs, at least including the direction of gravity relative to thestructural member of the patient's anatomy, and performs computations togenerate outputs, including the expected soft tissue position relativeto the structural anatomical member. Inputs to the software module mayinclude any parameter that helps predict the position of the softtissues with respect to the structural member of the patient's anatomy.The expected position of the soft tissues is governed by the mechanicsof the patient's anatomy, including environmental factors (e.g. thedirection of gravity).

The mechanics include forces, displacements, pressures, etc. within thepatient's anatomy. The computer model of the mechanics of the softtissue may be implemented in software, for example, as a finite elementmodel, a look-up table, an anatomical atlas, a differential equationsolver, a machine learning algorithm, etc. Any software implementationthat predicts, with sufficient accuracy for the surgical procedure, theposition of the soft tissues may be used.

The inputs may include a medical image of the patient's anatomy,including the structural member and the soft tissues of interest.Another input may include the direction of gravity during imageacquisition relative to the medical image, expressed as a vector in theimage coordinate system. The medical image may be a raw medical image(such as an MRI or computed tomography (CT) scan) or a fused imagecombining multiple modalities. Image segmentation may be used todifferentiate the various tissues within the medical image—for example,the structural anatomical members, the soft tissues, including regionsof interest within the soft tissues (e.g. a region of interest may be atumour or lesion within the brain, within the skull). Image segmentationmay be performed by a separate computing unit, and a segmented image maybe provided to the software module as an input. Alternatively, thesoftware module may perform image segmentation, where an unsegmentedimage is provided as an input.

Pressure (e.g. within a cavity containing the soft tissues) may beprovided as an input. For example, during a cranial procedure, thepressure within the skull may change as a result of a craniotomy. Thispressure (or pressure change) may be provided as an input to thesoftware module based on measured, predicted, a priori known, etc.values. The location of the craniotomy may also be provided as input.

Any of the previously described inputs, or any other input may be usedin any combination to generate the output of the software module.

The output of the software module is the expected position of the softtissues relative to the structural member of the patient's anatomy (towhich a reference element is coupled). The output may include theposition of the soft tissues, where this position is expressed aspositions of various connected nodes. The output may be the position ofa region of interest (e.g. the center of the region of interest). For acranial surgical procedure, the output includes brain shift. The outputmay also be in the form of adjusted medical images of the brainaccounting for movement due to the various inputs, including gravity.

FIG. 6 provides a flowchart of operations 600 which define a computerimplemented method, such as for a computing unit 104, to perform asurgical procedure on soft tissues. At step 602, the operations computea registration of a structural member of a patient's anatomy relative toa reference element coupled thereto. At step 604, the operationsreceiving inclinometer measurements from a sensor unit, the sensor unitcomprising an optical sensor to generate optical measurements from atracker and at least one sensor to provide inclination measurements,wherein the tracker is configured to provide positional information inup to six degrees of freedom to the optical sensor. At step 606, theoperations calculating a direction of gravity relative to the structuralmember of the patient's anatomy based on the inclinometer measurementsand the registration. At step 608, the operations determining spatialinformation of the soft tissues relative to the reference element basedon the registration, the direction of gravity and expected soft tissuemovement provided by a computer model modelling the mechanics of thesoft tissues of the anatomy responsive to the direction of gravity. Atstep 610, the operations receive optical measurements from the sensorunit of the tracker attached to a surgical tool, and compute a positionof the tool relative to the reference element based on the opticalmeasurements. And, at step 612, the operations provide surgicalnavigation data for display, the surgical navigation data based on theposition of the tool and the spatial information of the soft tissues.

The computer model of the anatomy may be based on any one or more of: amedical image of the anatomy; known material properties of tissues ofthe anatomy, including the soft tissues; a finite element model of theanatomy; a look-up table mapping expected soft tissue movement based onthe direction of gravity; blood pressure; and pressure within the softtissue cavity.

As noted previously, the reference element may be rigidly attached tothe structural member. The reference element may be non-invasivelyattached to the structural member. The method steps to calculate, todetermine, to receive optical measurements and to provide may beperformed in real-time. The structural member of the patient's anatomymay be a skull, and the soft tissues may be the patient's brain. Thereference element may be the sensor unit. The reference element may bethe tracker. The registration may be performed using a registration toolwith a tracker attached thereto.

The surgical navigation data may be relative to a surgical tool, wherethe surgical tool has a tracker attached thereto. Operations may furtherperform steps to receive medical image data; perform an imageregistration; and provide image guided surgical navigation for display.

FIG. 7 is a flow chart of operations 700 to perform a surgical procedureon soft tissues of a patient's anatomy. At 702, operations compute aregistration of a structural member of the patient's anatomy relative tothe reference element. A sensor unit comprises an optical sensor togenerate optical measurements from a tracker and at least one sensor toprovide inclination measurements to determine a direction of gravity.The tracker is configured to provide positional information in up to sixdegrees of freedom to the optical sensor. The reference element iscoupled the structural member.

At 704, operations calculate a direction of gravity relative to thestructural member of the patient's anatomy based on inclinationmeasurements and the registration. At 706, operations provide thedirection of gravity to a computer model modelling the mechanics of thesoft tissues of the anatomy; and, at 708, operations receive data fromthe computer model regarding a shift of the soft tissues of the anatomy.

The computer model may be provided by a remotely located computing unit(e.g. a server) relative to an intra-operative computing unit performingthe steps of operations 700. The surgical procedure may be a cranialprocedure and the shift may be a shift of a brain.

Operations 700 may provide surgical navigation data for display. Theshift may be presented such as via a display unit. Operations 700 mayreceive optical measurements, from the sensor unit, of the trackerattached to a surgical tool, and compute a position of the tool relativeto the reference element based on the optical measurements to provide assurgical navigation data. Operations 700 may further perform steps toreceive medical image data; perform an image registration; and provideimage guided surgical navigation for display.

In an exemplary system and/or in accordance with an exemplary computerimplemented method, a computing unit does not provide a computer modelof the soft tissue, but instead receives a vector representative of thedirection of gravity during imaging (i.e. during the generation of amedical image). The computing unit, based on a registration and/or imageregistration, may use inclination measurements of a reference element,and the vector representative of the direction of gravity during imagingto calculate the patient's current orientation with respect to thepatient's orientation during imaging, and provide this information to adisplay unit for communication to a user in real-time. The user may acton this information to bring the patient's anatomy into alignment withhow they were oriented (i.e. a similar or substantially similarorientation) during imaging. This may minimize the influence of gravitywhen performing surgical procedures with reference to the medical imageparticularly in surgeries where accuracy requirements are high. Theorientation of the patient during the procedure with respect to theorientation during imaging may be communicated by any means, includingnumerically, graphically (e.g. bubble level graphic), or otherwise. Thisinformation may be displayed prior to or during surgical navigation, andmay be displayed in real-time, such that any movement of the patientduring the surgery may be detected and the surgeon may re-adjust theposition of the patient based on the detected movement. Alternatively,the computing unit may cease to display surgical navigation if adifference in the orientation of the patient during the procedure andthe orientation of the patient during imaging exceeds a pre-setthreshold.

FIG. 8 is a flowchart of operations 800 which define a computerimplemented method, such as for a computing unit 104, to perform asurgical procedure on soft tissues. At 802, operations receive medicalimage data of a patient's anatomy including soft tissues, the patient'sanatomy being in a first known orientation with respect to gravityduring imaging.

At 804, operations compute a registration of a structural member of thepatient's anatomy relative to a reference element. A sensor unit,comprising an optical sensor generates optical measurements from atracker. The sensor unit also comprises at least one sensor to provideinclination measurements to determine a direction of gravity. Thetracker is configured to provide positional information in up to sixdegrees of freedom to the optical sensor. A reference element is coupledwith a structural member of a patient's anatomy.

At 806, operations calculate a second orientation with respect togravity of a patient's anatomy based on inclination measurements and theregistration. At 808, operations calculate the orientation differencebetween the first known orientation and the second orientation. And, at810, operations provide the orientation difference for display.

Operations may be configured such that the orientation difference isprovided as a bubble level graphic. Operations may provide surgicalnavigation of a surgical tool with respect to soft tissues. The surgicaltool may be coupled to a tracker and the sensor unit provide opticalmeasurements such that the operations determine the pose of the surgicaltool, in real time, during the procedure. Operations may persistentlydisplay the orientation difference during surgical navigation.Operations may not display the surgical navigation when the orientationdifference exceeds a threshold. The orientation difference may becalculated and displayed in real time during surgical navigation.

In another exemplary system and/or in accordance with another exemplarycomputer implemented method, a computing unit does not provide acomputer model of the soft tissue, nor does the computing unit receive avector representative of the direction of gravity during imaging. Thecomputing unit executes instructions providing to a display unit thecurrent direction of gravity with respect to the patient (e.g. shown asa vector overlaid on an image-guided display, such as the one depictedin FIG. 3). This display provides the surgeon with knowledge about thedirection of gravity relative to the patient's anatomy during surgery.This knowledge may allow the surgeon to adjust a surgical plan (forexample, based on the direction of gravity, the surgeon may decide toperform a more aggressive excision of a tumour around its margin facingthe direction of gravity, in anticipation that the tumour has shiftedslightly in that direction).

In the systems and methods described herein, measurements of thedirection of gravity with respect to the reference element are used inreal-time for surgical navigation. In this way, if a patient's rigidanatomical structure or structural member (e.g. skull) moves during theprocedure (unintentionally, or due to repositioning), the currentreal-time direction of gravity is used. The computing unit shown anddescribed here may take different forms. The computing unit may comprisea single computing device (e.g. laptop, workstation, tablet, etc., ormultiple computing devices (a computing device with a server, etc.).Medical image data may be accessed by various technologies including anetwork connection to imaging database, a USB key, a direct connectionto imaging equipment, etc.

The computing device may receive input from one or more user inputdevices including but not limited to a keyboard, a mouse or otherpointing device, a touch screen or other gestural interface, amicrophone for audio (voice) commands, etc.

A person skilled in the art will realize that the specification isapplicable to other forms of surgery and is not meant to be limited tobrain surgery. It is further understood that various methods describedfor performance by a computer system such as navigational surgery may beimplemented in software such as instructions and data to configure atleast one processing unit of a computer system to perform the method.The instructions and data may be stored in a device such as a memory(RAM, ROM, flash drive, etc.) or other non-transitory storage device(e.g.: magnetic, optical, or other disk or storage medium).

Accordingly, it is to be understood that this subject matter is notlimited to particular embodiments described, and as such may vary. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto be limiting.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the teachings herein. Any recitedmethod can be carried out in the order of events recited or in any otherorder which is logically possible.

The invention claimed is:
 1. A system to perform a surgical procedure onsoft tissues comprising: a tracker; a sensor unit, comprising an opticalsensor to generate optical measurements from the tracker and at leastone other sensor to provide inclination measurements to determine adirection of gravity, wherein the tracker is configured to providepositional information in up to six degrees of freedom to the opticalsensor, wherein one of the sensor unit or the tracker is adapted tocouple with a structural member of a patient's anatomy, and wherein thesoft tissues have a nominal position relative to the structural memberof the patient's anatomy; and a computing unit configured to: compute aregistration of the structural member of the patient's anatomy relativeto the one of the sensor unit or the tracker coupled with the structuralmember; calculate the direction of gravity relative to the structuralmember of the patient's anatomy based on the inclination measurementsand the registration; determine spatial information of the soft tissuesrelative to the one of the sensor unit or the tracker coupled with thestructural member based on the registration, the direction of gravityrelative to the structural member, and expected soft tissue movementprovided by a computer model modelling mechanics of the soft tissuesresponsive to the direction of gravity relative to the structuralmember, wherein the spatial information of the soft tissues is anexpected position of the soft tissue relative to the structural memberof the patient's anatomy; and provide surgical navigation data fordisplay.
 2. The system of claim 1, wherein the one of the sensor unit orthe tracker adapted to couple with the structural member is adapted torigidly attach to the structural member.
 3. The system of claim 1,wherein the one of the sensor unit or the tracker adapted to couple withthe structural member is adapted to non-invasively attach to thestructural member.
 4. The system of claim 1, wherein the computing unit,in real-time, calculates the direction of gravity relative to thestructural member of the patient's anatomy, determines the spatialinformation of the soft tissues, and provides the surgical navigationdata.
 5. The system of claim 1, wherein the structural member of thepatient's anatomy is a skull, and wherein the soft tissues are a brainof the patient's anatomy.
 6. The system of claim 1, wherein the sensorunit is adapted to couple with the structural member of the patient'sanatomy.
 7. The system of claim 1, wherein the tracker is adapted tocouple with the structural member of the patient's anatomy.
 8. Thesystem of claim 1, wherein the registration is performed using aregistration tool with the tracker or another tracker attached to theregistration tool.
 9. The system of claim 1, wherein the surgicalnavigation data is relative to a surgical tool, wherein the surgicaltool has the tracker or another tracker attached to the surgical tool.10. The system of claim 1, wherein the computing unit is furtherconfigured to: receive medical image data; perform an imageregistration; and provide image guided surgical navigation data fordisplay.
 11. The system of claim 1, wherein the expected position of thesoft tissue is expressed as positions of connected nodes.
 12. The systemof claim 1, wherein the expected position of the soft tissue isexpressed as a position of a region of interest of the soft tissue. 13.The system of claim 1, wherein the expected position of the soft tissueis expressed as adjusted medical images accounting for movement of thesoft tissue due to the gravity.
 14. The system of claim 1, wherein thesoft tissue is located within the structural member.
 15. The system ofclaim 14, wherein the soft tissue comprises a brain and the structuralmember comprises a skull.
 16. A computer implemented method to perform asurgical procedure on soft tissues comprising steps of: computing aregistration of a structural member of a patient's anatomy relative to asensor unit coupled to the structural member, wherein the soft tissueshave a nominal position relative to the structural member; receivinginclination measurements from the sensor unit, the sensor unitcomprising an optical sensor to generate optical measurements from atracker and at least one sensor to provide inclination measurements,where in the tracker is attached to a surgical tool, and wherein thetracker is configured to provide positional information in up to sixdegrees of freedom to the optical sensor; calculating a direction ofgravity relative to the structural member of the patient's anatomy basedon the inclination measurements and the registration; determiningspatial information of the soft tissues relative to the sensor unitbased on the registration, the direction of gravity relative to thestructural member, and expected soft tissue movement provided by acomputer model of the patient's anatomy modelling mechanics of the softtissues responsive to the direction of gravity relative to thestructural member, wherein the spatial information of the soft tissuesis an expected position of the soft tissue relative to the structuralmember of the patient's anatomy; receiving, from the optical sensor ofthe sensor unit, the optical measurements from the tracker; computing aposition of the surgical tool relative to the sensor unit based on theoptical measurements; and providing surgical navigation data fordisplay, the surgical navigation data based on the position of thesurgical tool and the spatial information of the soft tissues.
 17. Themethod of claim 16, wherein the computer model of the patient's anatomyis based on any one or more of: a medical image of the patient'sanatomy; known material properties of tissues of the patient's anatomy,including the soft tissues; a finite element model of the patient'sanatomy; a look-up table mapping the expected soft tissue movement basedon the direction of gravity relative to the structural member; bloodpressure; and pressure within a cavity containing the soft tissue. 18.The method of claim 16, wherein the sensor unit is rigidly attached tothe structural member.
 19. The method of claim 16, wherein the sensorunit is non-invasively attached to the structural member.
 20. The methodof claim 16, wherein the method calculates, in real time, the directionof gravity relative to the structural member of the patient's anatomy,determines the spatial information of the soft tissues, and provides thesurgical navigation data.
 21. The method of claim 16, wherein thestructural member of the patient's anatomy is a skull, and wherein thesoft tissues are a brain of the patient's anatomy.
 22. The method ofclaim 16, wherein the soft tissue is located within the structuralmember.
 23. The method of claim 22, wherein the soft tissue comprises abrain and the structural member comprises a skull.